Genetic and geographic structure of an insect resistant and a susceptible type of Barbarea vulgaris in western Europe

Abstract

Interactions between herbivores and plants are believed to have been important drivers of biodiversity. However, to drive an initial resistance divergence into different evolutionary lineages and taxa, these interactions have probably been embedded in other processes of divergence, like allopatric isolation. The cruciferous plant Barbarea vulgaris ssp. arcuata occurs in Denmark in two types: one (G) is resistant to most genotypes of the flea beetle Phyllotreta nemorum, the other (P) is susceptible. The two types additionally differ in hairiness and glucosinolates, they are genetically strongly divergent, and reproduction between them is reduced. To determine whether the two plant types and their resistance polymorphisms are also present outside Denmark, and to understand how they have evolved, we analysed 33 European populations of B. vulgaris for resistance, hairiness, glucosinolates, and microsatellite markers. Most populations had traits indicative of the G type, including the already characterized Danish G populations. In contrast, only two populations outside Denmark were of the P type; one from northern Sweden and one from Estonia. Genetically, the G populations formed two genetic clusters that were strongly divergent from a genetic cluster containing P populations. A fourth genetic cluster, which contained only a single population and no Danish plants, belonged morphologically to the subspecies ssp. vulgaris. The divergence found in Denmark between a resistant G and a susceptible P type is thus part of a larger divergence in Europe. Judging from the trait correlations, genetic divergence, and partial reproductive incompatibility, the plant types must have been isolated from each other for quite some time. The two P populations outside Denmark came from the north and east, suggesting a more eastern distribution. If so, resistant and susceptible types could have diverged during the ice age and later met in Scandinavia. However, more samples from Eastern Europe are needed to clarify this.

Natural and human‐mediated selection in a landrace of Thai rice (Oryza sativa)

“…The genetic structure of Muey Nawng appears to be much more influenced by genetic differentiation between gall midge‐resistant and susceptible types. A similar pattern is found in flea beetle‐resistant Barbarea vulgaris populations across Denmark where resistant and susceptible populations are clustered into separate genetic groups (Hauser et al , ). Studies of two Silene species across a large geographical scale showed not only structure due to resistance and susceptibility to anther smut fungus ( Microbotryum ), but also revealed geographical structuring across Europe (Vercken et al , ).…”

Multiple hydroxyphenethyl glucosinolate isomers and their tandem mass spectrometric distinction in a geographically structured polymorphism in the crucifer Barbarea vulgaris

“…2). However, occasional P-type populations in S. Scandinavia have been described in earlier papers from our research group (Agerbirk et al, 2001;Toneatto et al, 2010; ( Hauser et al, 2012;Christensen et al, 2014), suggesting that the Ptype may occur far into the apparent G-type dominated area at lower frequencies.…”

“…As the deviating P-type bears no clear resemblance to other tested Barbarea species (Agerbirk et al, 2003;Toneatto et al, 2012), the second hypothesis has no experimental support yet. In contrast, the theory of past allopatric separation has been supported by a recent demonstration of different geographic distributions of the two types: based on the characteristic genetic profiles of the P and G-types, the P-type was found to be widely distributed in Eastern Europe in contrast to a more western distribution of the G-type (Christensen et al, 2014;Hauser et al, 2012). Resistance properties and metabolite analysis of most of the accessions generally agreed with the genetic assessment of plant type status based on five unlinked microsatellite markers, but GSL data of two accessions were atypical.…”

The cytochrome P450 CYP72A552 is key to production of hederagenin‐based saponins that mediate plant defense against herbivores

“…Within the large crucifer family (Brassicaceae), only a few species in the genus Barbarea are known to produce saponins (Badenes-Perez et al, 2014). One of these species, Barbarea vulgaris, has diverged into two different 'plant types' that differ in saponin profile and herbivore resistance, as well as in other defense-related traits Hauser et al, 2012;Toneatto et al, 2012). One type, the G-type (Glabrous), is resistant to some specialist insect herbivores, including the devastating agricultural pest diamondback moth (Plutella xylostella) and the flea beetle (Phyllotreta nemorum).…”